Oil shale is found throughout the world and would constitute a plentiful and relatively inexpensive fuel if techniques were available for quickly and inexpensively processing the oil shale into combustible gases. One approach to processing oil shale into combustible gases is disclosed in U.S. Pat. No. 4,211,606 (the disclosure which is hereby incorporated by reference). In this patent, oil shale is heated in a dryer using clean, hot flue gases producing heated shale that is applied to a pyrolyzer. The heated shale is further heated in the pyrolyzer with hot ash to produce combustible products, and carbonaceous material that is added to a gasifier. Hot gases and steam are applied to the gasifier such that combustible gases are produced. The residue of the gasifier is extracted and applied to what the patent terms an air jet furnace, details of which are disclosed in U.S. Pat. No. 4,110,064 which is also incorporated by reference.
The air jet furnace produces combustion products in the form of hot flue gases whose major constituent is carbon dioxide, and ash particulate which is applied to a separator which separates the combustion products into a stream of hot coarse ash that is supplied to the pyrolyzer, and a stream of hot gas containing fine ash. The stream of hot gas and fine ash is applied to a separator that produces a stream of fine ash that is applied to the gasifier, and a stream of gases containing residual ash. The latter stream is applied to a further separator that produces the clean flue gases that serve to heat the shale in the dryer.
A less complex derivative of the apparatus described above has apparently been used in two plants in the U.S.S.R. in 1990 and 1991. As presently understood, the actual design eliminates the gasifier and the dryer. Oil shale is fed into a pyrolyzer wherein pyrolyzation takes place producing carbonaceous material after a predetermined residence time of the shale in the pyrolyzer. This material is supplied to an air jet furnace wherein combustion takes place producing hot flue gases, and particulate that is applied to a separator which separates the flow into a stream of coarse ash, and a stream of hot flue gases containing fine ash, such as fly ash. The stream containing the hot coarse ash is applied to the pyrolyzer which produces pyrolysis gas at a temperature in excess of 400.degree. C. Such gas contains combustible products, steam and carbon compounds. The stream containing the combustible products from the pyrolyzer is applied, together with the stream of hot flue gases containing fine ash to a burner that is part of a combustion chamber of a boiler that produces steam that may be used for generating electricity.
The problem with a power plant utilizing this technique for burning oil shale is a reduction in thermal efficiency and available power caused by fouling of the heat transfer surfaces in the boiler caused by the entry of fly ash into the boiler, and by carbonate decomposition which results in increased energy consumption and increased carbon dioxide emission from the power plant. The air jet furnace must operate at high temperatures to obtain process stability. Such stability is achieved when the temperature at the output of the furnace is high enough so that the coarse ash added to the pyrolyzer has sufficient heat to both heat the oil shale in the pyrolyzer as well as to evaporate water and organic matter associated with the shale. It has been estimated that more than about two-thirds of the amount of fly ash entering the boiler is derived from the fine ash carried with the hot gases from the air jet furnace, and less than about one-third is derived from the combustible gases produced by the pyrolyzer.
Reducing the temperature of the furnace will reduce the amount of carbonate decomposed therein and improve carbondioxide emission from the power plant, but such reduction in temperature is achieved at the cost of increasing the circulation rate through the furnace which has the detrimental effect of increasing the parasitic losses of the power plant and consequent reduction in the overall efficiency of the plant. In addition to this, the usual problem of fouling of the heat exchange surfaces is still present requiring periodic mechanical cleaning with high pressure liquid to remove soft deposits, and complete shut down and manual scraping to remove hard deposits.
It is therefore an object of the present invention to provide a new and improved method of and means for producing combustible gases from low grade solid fuel which substantially overcomes or significantly reduces the disadvantages outline above.